1. Field
Embodiments relate to power allocation in Wireless multiple-input-multiple-output (MIMO) systems with base stations having relatively large antenna arrays.
2. Description of the Related Art
Wireless Time Division Duplex (TDD) multiple-input-multiple-output (MIMO) systems represent an advance in wireless communication. MIMO systems employ two or more antennas at the transmitting and/or receiving ends of a wireless link. The multiple antennas improve data transmission rates, while holding radio bandwidth and power constant.
A MIMO transmitter transmits an outgoing signal using multiple antennas by demultiplexing the outgoing signal into multiple sub-signals and transmitting the sub-signals from separate antennas. MIMO exploits the multiple signal propagation paths to increase throughput and reduce bit error rates. Using MIMO techniques the rate of transmission increases linearly depending on the local environment.
A typical TDD wireless system may service a relatively large geographic area organized in relatively smaller geographic units known as cells. Each cell includes a base station serving n mobiles also called mobile users, or user equipments, etc. All user equipments in all cells send pilot signals to the corresponding base stations. In each cell n user equipments send orthogonal pilot signals vk, k=1, . . . , n to the associated base station.
As is known, when the number of antennas associated with a base station in a MIMO system becomes very large the performance of the system virtually stops depending on additive noise, which is necessarily present in the receiver of each base station antenna (e.g., electronic equipment associated with each antenna which may be processing signals arrived to that antenna). However, increasing data transmission rates becomes difficult due to inter-cell interference caused by contamination of downlink signals transmitted to user equipment located in different cells.
Contamination of downlink signals is caused by the contamination of pilot signals arriving at a base station from user equipment located in different cells.
The contamination of the pilot signals is unavoidable due to reuse of the pilot sequences by user equipments associated with base stations in different cells. In other words, if a k-th user equipment associated with a base station in cell 1 uses pilot vk and in a neighboring cell 2 there is a user equipment associated base station in cell 2 that also uses pilot vk, then the downlink signal transmitted from the base station in cell 1 to the k-th user in cell 1 interferes with the downlink signal transmitted from the base station 2 to the k-th users in cell 2.